I leave mine in the sound for the winter near anacortes. so similar conditions to you. the seawater will keep the boat a little warmer than freezing but you can have challenges if its cold for a long time and items are way from the submerged hull. this means the big question is whether you put antifreeze in the fresh water tank. which has the downside of tasting of antifreeze for a while come spring ... some people put alcohol in instead.
I have a engine room heater.
https://www.westmarine.com/buy/caframo--pali-bilgesafe-heater-400w-with-thermostat--12960282
I have two dehumidifier heaters in the cabin.
https://www.westmarine.com/buy/west...fan-dehumidifier-120v-ac--7867518?recordNum=4
first year I left the freshwater tanks full. no tank issues. But the droplets of residual water in the galley and outside sink deformed the spray heads. Now I just unscrew these and remove come October - will do again this year.
second year the boat was serviced in Jan so it was on the hard. they drained the freshwater system (you can just move the over pressure switch on the hot water tank manually and it will pump out by itself into the bilge). they didnt put antifreeze into the tank.
I will also pour a little antifreeze into the shower sump via the shower drain and also the waste tank via the toilet.
I keep the diesel tank full to avoid condensation int he tank - getting water into the fuel.
keep the seacock closed when not in use.
I can still use the boat in the winter - just no fresh water. unplug shore power and lift out the caframo from the engine bay. (dehumidifiers stay plugged in as the 110v circuit is off anyway - when off shore power. )
for the engine heater, some folks mount the caframo on the bulk head and run the wiring properly. I dont do this, I just let it sit on the high platform in the engine room (out of the bilge) and run the cable over the engine and through the gap where the fuel tank access is into cabin where I plug it into a 110v socket above the oven (cutwater). takes 2 mins to remove if I want to take the boat out. I open the engine door anyway to turn on the seacock so its not a big deal.
Come the spring its removed for the season.
There are some beautiful boating days in the winter, I dont want to give these up with a full winterizatoin and we dont have to here in BC/PNW. not like those poor midwest /east coast boaters who shrink wrap and store boats in heated sheds
😀